An apparatus of the above kind is described in French patent specification No. FR 2 713 279A. This known apparatus enables the rate at which the temperature of the engine is raised to its working temperature to be increased after a period during which the engine is stopped, when the temperature of the heat transfer fluid contained in the reservoir is still at a relatively high temperature at the time when the engine is restarted. However, although the thermal insulation on the reservoir reduces heat losses to the surrounding atmosphere, and even when valves, which are disposed on either side of the reservoir, prevent any heat transfer by convection between the coolant fluid contained in the reservoir and that contained in the circuit close to the reservoir, transfer of heat, by conduction across the above mentioned valves, remains possible within the fluid.